Conventional transmitters for various purposes may share a common medium for transmission. The medium may include a portion of the electromagnetic spectrum in free space or in one or more conductors (e.g., electrical, magnetic, or optical). Generally, reception of transmitted signals may be impeded when transmissions overlap, for example, by occupying the same portion of the electromagnetic spectrum at substantially the same period of time as detected by any receiver coupled to receive from the medium. Overlapping signals may be distinguishable if different in received signal strength or in modulation. However, in a medium where transmitters of the same or different systems are not otherwise controlled to reduce overlap, the reliability of each system using the common medium may be degraded when the likelihood of overlap increases. Overlap becomes more likely, generally, as the number of transmissions in a period of time increases.
Systems designed for high reliability include systems relied upon for reducing the risk of loss of equipment and loss of human life. The problems described above present serious reliability, security, and economic limitations on the use of one or more systems having transmitters that share a common medium. For example, aircraft collision avoidance systems share free space for transmitting interrogations and replies in portions of the radio spectrum designated by government regulations. One such system, commonly known as a traffic collision avoidance system (TCAS) operated in an aircraft in flight includes methods for transmitting at limited frequencies, times, and power levels to reduce interference among other TCAS equipped aircraft, conventional air traffic control systems, and navigation systems. Transmission power levels are conventionally limited according to a so called Interference Limiting technique and according to a Whisper-Shout technique, both of which being well known and specified in DO-185A “Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System II (TCAS II)” available from RTCA Inc. However, substitute and additional methods for determining suitable transmission power levels are needed to further improve the reliability of TCAS systems so as to reduce the risk of economic losses and the risk of loss of human life. Methods according to the present invention and systems that employ these methods improve the reliability of any systems that share any common medium.